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  • Recent developments in superhydrophobic surfaces and their relevance to marine fouling: a review

    JAN GENZER & KIRILL EFIMENKO

    Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina, USA

    (Received 12 May 2006; accepted 22 August 2006)

    Abstract In this review, a brief synopsis of superhydrophobicity (i.e. extreme non-wettability) and its implications on marine fouling are presented. A short overview of wettability and recent experimental developments aimed at fabricating superhydrophobic surfaces by tailoring their chemical nature and physical appearance (i.e. substratum texture) are reviewed. The formation of responsive/‘‘smart’’ surfaces, which adjust their physico-chemical properties to variations in some outside physical stimulus, including light, temperature, electric field, or solvent, is also described. Finally, implications of tailoring the surface chemistry, texture, and responsiveness of surfaces on the design of effective marine fouling coatings are considered and discussed.

    Keywords: Marine fouling, wettability, superhydrophobic surfaces, responsive/‘‘smart’’ surfaces, amphiphilic surfaces

    Introduction

    Questions frequently asked are ‘‘why is it so difficult

    to design an antifouling (AF) surface’’ and ‘‘why can-

    not the design of an ‘optimal’ AF surface be based on

    what is already know about wettability?’’ This is

    because there the various surface-modification meth-

    ods capable of fabricating both non-stick and very

    sticky surfaces are well-known. For instance, it is

    known that frying pans have to be coated with Teflon

    in order to make them non-stick. Gortex (a specific

    version of a Teflon-like material) raincoats protect

    the wearer during rainy days. The opposite of wett-

    ability, namely very wettable surfaces, is also well

    known. For example, before painting a house, a

    primer is typically applied, which enables facile

    application of the final coating layer. In their quest

    to develop effective AF coatings, researchers soon

    realised that much more was required than applying

    a high quality layer of Teflon. Initial insight into this

    complex issue can be seen by surveying the partition

    of proteins at surfaces (Norde, 1996; Latour, 2004

    and references therein). Being composed of hydro-

    phobic cores and hydrophilic coronas, proteins

    typically partition relatively readily on both hydro-

    philic and hydrophobic surfaces. The quantity of

    adsorbed protein is regulated by the conditions of

    the surrounding solution; it is highest close to the

    protein’s isoelectric point, where charges from

    neighboring proteins are effectively eliminated.

    Proteins can physisorb on hydrophilic surfaces via

    attachment of their coronas to the substratum. When

    in contact with hydrophobic materials, proteins can

    ‘‘open up’’ and place their hydrophobic segments

    directly on the surface. The latter phenomenon leads

    typically to protein denaturation, i.e. adsorption into

    some irreversible conformational state, from which

    proteins cannot recover readily. This simple example

    illustrates that wettability itself, at least at its very

    extremes, cannot aid the design of an efficient

    protein-repellent surface. Indeed, it has now been

    appreciated that it may not be wettability itself,

    but rather the structure of water molecules near

    the substratum, which may help in the design of

    protein-resistant surfaces. Ethylene glycol-based

    surfaces represent examples of such materials

    (Mrksich & Whitesides, 1996). They can effectively

    bind water molecules and prevent intervening pro-

    teins from replacing them, hence making them

    protein adsorption-resistant.

    When extending this simple example (the empha-

    sis being on ‘‘relatively simple’’ as many outstanding

    issues still remain in designing effective protein-

    resistant surfaces) to more complex cases involving

    Correspondence: Jan Genzer, Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA.

    E-mail: Jan_Genzer@ncsu.edu

    Biofouling, 2006; 22(5): 339 – 360

    ISSN 0892-7014 print/ISSN 1029-2454 online � 2006 Taylor & Francis DOI: 10.1080/08927010600980223

  • biomaterial adsorption, the complexity of the pro-

    blem can be immediately appreciated. One of the

    issues is the fact that almost any biomass is made of

    hydrophobic, hydrophilic, and charged components.

    Moreover, these bio-moieties are adaptable (or

    ‘‘smart’’); they can adjust their state to the adsorbing

    medium very rapidly and efficiently. This, obviously,

    makes the task of designing an efficient AF surface

    very challenging.

    In order to conceive an optimal foul resistant

    surface, the driving forces that govern the partition

    of biomass on man-made surfaces first have to be

    identified and controlled. Wettability is a key para-

    meter that needs to be tailored. As will be discussed

    below, wettability is intimately related to both

    chemical constitution and the physical topology of

    surfaces. There are many examples in nature, some

    of which are discussed below, where wettability due

    to ‘‘chemistry’’ is fine-tuned by additional topology

    effects. Another important issue in creating effective

    functional AF surfaces is the ability of surfaces to

    change their appearance in response to some external

    trigger. The ability of surfaces to respond to varia-

    tions in outside stimuli will depend crucially on

    how fast reconstruction events take place on those

    surfaces. Clearly, a very complex set of issues

    involving various molecular phenomena, which are

    mutually intermingled, are involved (Anderson et al.

    2003).

    The authors are not proposing to solve the lasting

    problem of biofouling in this paper. Instead, the

    review will highlight recent experimental develop-

    ments aimed at understanding wettability of materi-

    als. Particular emphasis will be on non-wettable

    surfaces as they provide informative insight about

    how the structure of the surface influences the parti-

    tioning of the liquid phase. The experimental findings

    will be put in the context of recent theoretical models.

    Some recent case studies will also be reviewed, which

    are aimed at designing so-called responsive/‘‘smart’’

    surfaces, structures that change their characteristics

    as a result of some external stimulus, such as light,

    temperature or wettability. Finally, some outstanding

    issues relevant to the rational design of an effective

    AF surface will be outlined.

    Wettability ‘‘101’’

    Since many excellent reviews dedicated to this topic

    have appeared recently (Feng et al. 2002; Blossey,

    2003; Callies & Quéré, 2005; Sun et al. 2005a;

    Marmur, 2006a; 2006b; 2006c) only a brief account

    of some of the outstanding phenomena in the field

    will be discussed. Wettability represents a funda-

    mental property of any material; it reveals informa-

    tion about the chemical structure of the material and

    its surface topology. However, as will be discussed

    below, decoupling these two effects is not always

    straightforward (and in some instances nearly im-

    possible) to do.

    More than 200 years ago, the English physician

    Thomas Young, identified in a recent biography as

    ‘‘the last man who knew everything’’ (Robinson,

    2006), described the forces acting on a liquid droplet

    spreading on a surface (cf. Figure 1a). The so-called

    contact angle (y) of the drop is related to the inter- facial energies acting between the solid-liquid

    (gSL), solid-vapor (gSV) and liquid-vapor (gLV) inter- faces via:

    cos ðyÞ ¼ gSV � gSL gLV

    ð1Þ

    The expression given by Equation 1 is a clear

    oversimplification of the real situation as it is strictly

    valid only for surfaces that are atomically smooth,

    chemically homogeneous, and those that do not

    change their characteristics due to interactions of the

    probing liquid with the substratum, or any other

    outside force. Any real surface exhibits two contact

    angles, so-called advancing (yADV) and receding (yREC) contact angle. The difference between them, referred to commonly as the contact angle hysteresis

    (CAH), is a measure of the surface ‘‘non-ideality’’

    (Gao & McCarthy, 2006c). As will be discussed

    below, the CAH is intimately related to the adhesion

    of materials on surfaces. Depending on the value of y, as measured by water, so-called hydrophilic (y5 908) surfaces can be distinguished from hydrophobic

    (y4 908) surfaces. Extremes to those two categories are superhydrophilic and superhydrophobic surfaces.

    The latter category is particularly interesting as it

    characterises surfaces that are nearly completely non-

    wettable (typically taken as y4 1508). Depending on the level of surface roughness two different regimes

    can be distinguished. In the so-called Wenzel regime

    (Wenzel, 1936; cf. Figure 1b),

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